[iOS] Should close Manage sync settings when sync is disabbled

Adding observer of the Manage sync settings to the sync service. If the
sync is disabled by the enterprise policy, the Manage sync settings
has to close itself.
This can happens right after sign-in or right after starting the app.

Bug: 937118
Change-Id: I4d8adac46472a3543969969283023146c8b559d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1508283
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641844}
1 file changed